The long-range intention is to determine principles that govern retention and forgetting in the developing rat. Particular attention is given to age-specific peculiarities in stimulus selection during learning and memory of the events of infancy. The experiments proposed address two general distinctions among theories to explain infantile amnesia. One set of theories emphasizes subsequent growth of several kinds, including the kind responsible for age-specific differences in what is stored in memory. The other set is split on the issue of the permanency of infantile amnesia; some theories expect ready alleviation of the amnesia, but others assume that alleviation is impossible without drastic physiological intervention. Most of the proposed experiments address the former issue--the effect of ontogenetic change in stimulus selection on learning and memory--but the latter also is addressed. The central thesis is that ontogenetically determined dispositions provide constraints on the developing rat as to what stimuli are "selected" for memory storage, with the impact of these constraints differing in accord with other environmental or physiological circumstances. Sets of experiments will focus on four specific aims: (1) To assess ontogenetic differences in the integration of temporally disparate events, toward understanding of how stimulus selection is determined by the temporal distribution of potentially organizable but physically separate events; (2) To assess ontogenetic differences in how stimulus selection may be altered by the use of strong, biologically significant stimuli as unconditioned stimuli or reinforcers; (3) To determine the effects of arousal, differentiation among sensory modalities and stimulus factors on the ontogeny of stimulus selection; and (4) To assess the influence of stimulus selection on infantile amnesia and the alleviation of infantile amnesia.